We Removed the Compressors. You’re Welcome.

How we replaced the compressors in our ultra-low freezers with the free-piston Stirling engine — and why that’s a huge benefit for you

Unlike cascade systems, which use two compressors and cooling loops in a cascading arrangement, the free-piston Stirling engine uses a reciprocating piston and displacer, along with a continuous, gravity-driven thermosiphon to cool the cabinet interior.

How does the Stirling engine revolutionize ULT system reliability?

  • The advanced linear motor and thermosiphon system design is inherently more efficient and reliable, with only two moving parts.
  • The few moving parts never wear because gas bearings eliminate all physical contact (friction) during motion.
  • There is neither oil to clog up the system nor lubrication levels to maintain.
  • Continuous modulation controls temperature like cruise control controls speed in your vehicle. Compressor start/stop cycles would be akin to controlling your car’s speed by cycling the ignition on and off. This excess mechanical and electrical stress leads to failures and higher electrical usage.
  • The gravity-fed thermosiphon is isothermal, driving superior interior temperature uniformity without high pressure or pumps.
  • Minimal preventive maintenance is required, for reliable cooling operation.

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ENERGY STAR® Test Results

Test Conditions (°C) -80 -70
Door Test Cabinet Temperature
Energy Test Cabinet Temperature
Ambient Temperature – dry bulb (Ta3 Tb)*
Ambient Temperature – dry bulb (Ta3 Tb)
Energy Consumption Including 6 Door Openings (kWh/day)
Energy Consumption Without Door Opening (kWh/day)
(24.0, 23.7)
(18.5, 17.7)
(24.2, 24.0)
(18.4, 17.9)
Weighted Average Energy Consumption at -75°C
Including 6 door openings (kWh/day)
Without door openings (kWh/day)
*Ta is at a height of 85.0 inches above the ground and Tb is at a height of 37.5 inches above the ground.

SU780XLE: The first and best ENERGY STAR®-certified ULT storage solution

At -80°C, Stirling Ultracold ULT freezers use 70–75% less energy than standard compressor-based ULT freezers. Our SU780XLE upright model meets the ENERGY STAR certification without using a performance-reducing “eco” mode that trades sample safety for energy rating.

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Premium technology with the lowest cost of ownership

Have you considered what your current ULT freezers cost you in energy consumption, maintenance, and building HVAC and floor space? You will see significant savings of 40% per year over the freezer’s lifetime:

  • HVAC costs can be up to 60% of a lab’s operating costs. SU780XLE’s technology can reduce HVAC and electrical infrastructure requirements by > 40%.
  • Larger storage capacity per footprint can save up to 70% sample storage cost per square meter/foot of floor space.
  • Proven minimal annual maintenance and extremely low lifetime service costs with thousands of freezers in service.

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Unparalleled commitment to environmental sustainability

Our facility’s Zero Waste-Certification is not a mere symbol or marketing pitch; it’s a reflection of the core values upon which we were founded. We have always brought sustainability and energy efficiency to the research community because it is the right thing to do for the environment, and our Zero Waste facility is an extension of the products we provide. Every ultra-low freezer we manufacture supports climate action goals by using 100% natural refrigerants, while also delivering the industry’s lowest energy consumption and smallest operating carbon footprint.

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